Treatment of light metal alloys



Patented May 14, 1929.

. DIANE"), MICHIGAN, ASSIGHOR TO THE DOW CHEMICAL COM- luIIlDLAND, MICHIGAN, A. CURIORATION OF MICHIGAN.

TREATMENT OF LEG-HT METAL ALLOYS.

Ho brewing.

in my copending application, filed Nov. 28, 1921, Ser. No. 518,142, I described an improveinent in the art of making alloy articles having magnesium as a principal ingredient and containing one or more metals capable of dissolving in solid magnesium in larger amounts than are found in solution in the alloys as cast, such improvement consisting in subjecting such articles to a heat treatment at a high temperature but below the melting temperature of the most fusible constituent of the alloy. The result of the aforesaid treatment is to cause increased. solution of the other metal or metals in the solid magnesium, this being accompanied by a marked increase in the strength and/or plasticity of the articles. The increase in plasticity thus obtained in light metal alloy articles of the class in question is of particular im )ortance where it is desired subsequently to subject the article to plastic deformation, such for example, as to a forgin", drawing, rolling, or extruding operation. The articles thus treated are also found to be rendered susccptible to further improvement in certain physical qualities by aging at regulated temperatures.

The improvements herein claimed related to certain specific aspects of theprocess and resultant product described in my aforesaid application.

In other words, specifically different results may be obtained, depending upon the constitution of the alloy that is subjected to the aforesaid heat treatment, and, while a wider field of use will be found with magnesium-aluminum alloys, other binary as wellas ternary and more complex magnesium alloys are susceptible to marked improve ment by trcatn'ient in the fashion described.

Having regard to theaccomplishment of the foregoing and related ends, the invention then consists of the steps hereinafter fully described. and particularly pointed out in the claims, the following description setting forth butono of the various ways in which the principle of the invention may be used.

As indicated, above,the principle involved in my improved method of regulated heat treatment is specifically illustrated and described in my co-pending application by reference to a magnesium-aluininum alloy, the one selected by way of example containing from six to eight per cent aluminum. Upon subjecting such an alloy to a high temperature, but below the melting point of the most application filed January 13, 18W. Serial No. 181,029.

fusible constituent of the alloy, this ordinarily belng the eutectic, the tensile strength, as well as other physical properties, such as rigidity or plasticity, hardness, etc, may be matenally modified and the article rendered more sultable for various mechanical uses as well as adaptable for further fabrication The improvement obtained in articles subected to this regulated heat treatment is apparently due to the rearrangement of the mlcro structure of the alloy and particularly to an increase in the amount of solid solution of the minor constituent or constituents pres out at the expense of the eutectic. In other words, the latter, where it includes a properly selected metal or metals, may be caused gradually to dissolve with the result that an increased amount of themetal in question will be found in solid solution in the magnesium.

Even in the case of a magnesium alloy where the minor constituent is a metal such as cadmium, which in casting at once forms a maximum solid solution with the magnesium, heat treatment of the regulated character hereinbefore described may be advantageously employed, inasmuch as it will insure a more uniform distribution of the two metals in such solution, by eliminating cored crystals.

In order to set forth in inclusive fashion,

the different types of micro-structure obtain able in cast magnesium alloys, together with the effect of heat treatment of each such type in the fashion hereinbefore described, as also the temperature possibilities for heat treatment of the same, the following tables are herewith presented, the first relating to typical binary magnesium alloys and the second to certain selected ternary and more complex magnesium alloys.

.1. Heat treatment of binary magnesium alloys.

Y ag: &2? Typical Effect of heat Temperature for alloy alloys treatment heat treatment 1. Complete Mfr-Cd Eliminate cored Up to near melting solid solucrystals. point of solid sotion. lution. 2. Combination Mg-Al Increase amount Up to near melting of solid so- Mg-Zn of solid solution point of eutectic, lution and MeeSn at expense ofeuand when euteceutectic. tectic and elimitie is all disnate cored cryssolved, up to tnls. near melting point of solid solution. 3. Eutectic. Mg-Cu Relieve casting Up to near melting MgNi strains; may 111- point of eutectic.

so cause rearrangement of eutectic.

II. Heat treatment of ternary. chz, magnesium ulimately 1% and 0.5% respectively.

Aland Zn must be present in appreciable amounts-more than approximately 1% and 0.5% respectively.

Other possible combinations of alloying ingredients can be easily formed from the data of Tables I and II above. Four possibilities for obtaining a microstructure consisting of a solid solution with or without a eutectic by regulated heat treatment, all'ot which possibilities come under the second type of structure inthe foregoing tables, may be noted, viz (1) 1 binary magnesium alloys with eutectic and solid solution structure still retain after heat treatment both a eutectic and solid solution; binary magnesium alloys with eutectic and solid solution structure lose on heat treatment such eutectic structure, the minor constituent going entirely into solid solution with the magnesium; (3) ternary or more complex magnesium alloys in which at least part of the minor constituents form eutectic with the magnesium and, on heat treatment, one of these constituents is partly or wholly dissolved in the magnesium to form solid solution, while the other or others are present either as solid solution or eutectic; and 4) ternary or more. complex magnesium alloys in which at least part of the minor constituents form eutectic with the magnesium and such eutectics have different melting points and vary as to the ease with which they dissolve in the magnesium to form solid solution, and, on heat treatment, one or more. of these constituents are partly or wholly dissolved in the magnesium to form solid solution, while the other. or others, if any, are present either as solid solution or eutectic.

The differentiation between the foregoing possibilities in some cases is not great; for example, a 1nagnesiummluminum-zinc alloy not only comes under the third possibility, since it can be so heated that the zinc will dissolve and the aluminum form the basis of the eutectic, but, on the other hand, the same alloy clearly comes under the fourth possibility, since there is a di ll'ercncc in melting points between magncsiiun-aluminum eutectic and the magnesium-zinc eutectic, and the aluminum can be rcscnt both as sol id solution and the eutectic. Again. the n'iagnesium-aluminuni-copper alloy will come under both the third and fourth possibilities for there is a diil'crence in melt ing points between magnesium-aluminnm and magnesium-coppcr eutectics; the magnesium-alumimmi eutectic dissolves fairly easily while the magnesiuni-copper eutectic does not dissolve at all and the aluminum can be present both as solid solution and eutectic.

The possibilities as to change in structure obtainable by my improved method of heat treatment, where dealing with alloys of the composition coming under the first and third types in the foregoing tables. will it; is believed be suiliciently evident. from a consideration of the tabulated data without further detailed explanation.

As to the range of temperatures to be employed. this will vary as previously indicated with the composition of the alloy being treated. Thus the maximum temperature. should not exceed the meltingpoint of the more fusible constituent which will be the eutectic where present, or the lower melting eutectic if there be more than one. The Mg-Al eutectic. for example Inclt's at 824 F. and where such eutectic occurs, c. g. in an 8% aluminum alloy. a temperature of from 750 to 820 I will be desirably cnuiloyed. However, a lower temperature will sutlice, provided the heat trcatmci'it be suiiiciently prolonged. The Mg-Zn eutectic melts at (350 F. and the Mg-Sn eutectic at 1049" R, from which it may be readily ascertained at whattemperatures alloys wherein either of these eutectics is present may be most advantageously treated.

Referring to the various possibilities of heat treatment in eil'ecting micro-structural changes in alloys of the second class in Tables I and II above, it will be noted that in my aforesaid copcnding application in addition to claims of a more comprehensive character, I have claimed specifically the case (being the first possibility hereinbefore explained) where a binary magnesium alloy with eutectic and solid solution still retains after such heat treatment both a eutectic and solid solution. In contradistinction I herein claim the application of such heat treatment to such an alloy where the result is the loss or disappearance of the eutectic structure formed by one of the minor constituents with magnesium (being the second and third possibilities hereinbcfore explained). Subordinate to the foregoing, but also distinguishing from such co-pending application, claim is also herein made to the specific case where one of two minor constituents is capable and the other substantially incapable of forming a solid till lltl

solution with the magnesium and the heat treatment, causes an increased solution of such tirst-named constituent.

A typical binary magnesium base alloy to which the present specitic method of heat treatment is applicable is one wherein aluminum is the other constituent. It such an alloy. containing for example approximately 8% oi the minor constituent is subjected to a heat. treatment at from 800 to 820 F. for four hours, or more, the eutectic originally present will have practically if not entirely disappeared, indicating that all of said constituent has nope into solid solution with the magnesium. A typical, more complex alloy is one containing approximately 8.5% of aluminum and 2')? of copper, with or Without cadmium and/or zinc in still lesser amounts. respectively (0. g. from Oil-1.0%), the balance being magnesium. Upon heating such an alloy substantially as just described, the tlllll'lllllilll'l is caused to go from eutectic into solid solution form, but the eutectic formed of the copper remains, being possibly changed with respect to its arrangement; whereby additional b neficiating results may accrue, as in the relieving of casting, strains, etc. Cadmium it included as an ingredient in such last-mentioned alloy will of course be present. at all times, i. e., both before and after heat treatment, in the form of a solution; while the zinc, if present initially in an amount sutliciently large to cause it to be in cluded in the eutectic structure, will in gene 'al behave in the same manner as the aluminum, noting, however, that its eutectic has aconsiderably lower melting point and that the heat treatment should accordingly be in graduated stages.

The following tables have been constructed in order graphically to illustrate the etl'ect of my improved method of regulated heat treatment upon allows having the ditlerent types of structure hereinbet'ore set forth, viz

III. (lost structures.

.Solid solution and eutectic Solid solution and eutectic Eutectic and/or solid solution{ In the foregoing table, the alloy as cast may have a structure indicated at the starting end ol the several arrows or by alloying certain other metals with the alloy having such starting structure result in the structures indicated at the end of the arrows being obtained. As indicated above, there exists a considerable number of possible changes in Solid solution and eutectic Solid solution and eutectic Solid solution itnd eutectic Solid solution and eutectic Eut ctic and/or solid solution structure where. different metals constitute the minor alloying constituent or constituents with magnesium torn'iing the base, depend ing upon the number of such ingredients and the particular metals selected. The following table will serve graphically to illustrate the results obtained by heat treating the structures indicated above, viz

IV. Effect of regulated heat treatment.

Cast

(1) Solid solution eutectic (2) Solid solution -ieutectic Ileat treated Solid solution (increased) eutectic (decreased) Solid solution Solid solution eutectic {Solid solution (increased) eutectic (decreased) Eutectic and/or solid solution l-lutcctic and/or solid solution {Folid solution Eutectic and/o1 solid solution $01M {:nlufifln (Home {Solid solution (increased) eutectic (decreased Solid solution eutectic solid solution eutectic Solid solution Solid solution eutectic t olid solution Solid solution (increased) eutcctic (decreased) Solid solution Solid solution (4) solid solution eutectic [Eutectic and/or solid solution Solid solution eutectic Eutectic and/or solid solution Solid solution Solid solution eutectic Eutectic and/or solid solution Solid solution Solid solution (increased) eutectic (decreased) Eutectic and/or solid solution Solid solut ion {Solid solution Eutectic and/or solid solution The term eutectic, asherein used, it will be understood, includes not only the wellknown ordinary eutectics with their more or less characteristic lamellar or globular structure, but also individual more or less well-defined metallic or intermetallic crystals which so often occur with relatively small percentages of added constituents.

Other modes of applying the principle of my invention may be employed instead of the one explained, change being made as regards the com osition and method herein disclosed, provide the ingredients or steps stated by any of'the following claims or the equivalent of such stated ingredients 01' steps be employed.

I therefore particularly point out and distinctly claim as my invention 1. In the art of making alloy articles havingmagnesium as the predominant constituent and also containing as a minor constituent a metal normally forming both a eutectic and a solid solution with the magnesium, the step which consists in subjecting such article to a heat treatment at a high temperature, but below the melting point of such eutectic, to cause substantially complete solution of the latter. I

2. In the art of making alloy articles having magnesium as the predominant constituent and also containing aluminum as a minor constituent in amount sufiicient normally to form both a eutectic and a solid solution with the magnesium, the step which consists in subjecting such article to a heat treatment a at an elevated temperature, but below the melting point of such eutectic, to cause substantially complete solution of the latter.

3. In the,art of making alloy articles havmg magnesium as the predominant constituent and also containing as minor constituents a metal capable and another substantially incapable of forming a solid solution with the magnesium, the step which consists in subjectin such an article to a heat treatment at a big temperature, but below the melting point of the most fusible constituent of the alloy, to cause increased solution of such first minor constituent and still leave a definite eutectic structure including such second minor constituent.

4. In the art of making alloy articles having mitgnesium as the predominant constituent an also containing aluminum and copper as minor constituents, the step which consists in subjecting such an article to a heat treatment at a high temperature, but below the melting point of the most fusible constituent of the alloy, to cause increased solution of the aluminum in the magnesium and still leave a 'definite eutectic structure including the copper.

5. In the art of making alloy articles having magnesium as the predominant constituent and also containing as minor constituents a metal ca able and another substantially incapable of forming a solid solution with the ma esium, the ste which consists in subjectmg such an artic e to a heat treatment at a high temperature, but below the meltin point of the most fusible constituent of the afioy, to cause substantially complete solution of such first minor constituent and still leave adefinite eutectic structure including such second minor constituent.

6. In the art of making alloy articles having magnesium as the predominant constituent and also containing aluminum and cop Jer as minor constituents, the step which consists in subjecting such an article to a heat treatment at a high temperature, but below the melting point of the most fusible constituent of the allo to cause substantially complete solution 0 such first minor constituent and still leave a definite eutectic structure includin such second minor constituent.

As an article of manufacture, a heat treated light metal alloy article, wherein magnesium is 'the predominant constituent, the alloy containing as minor constituents a metal capable and another substantially incapable of forming a solid solution with the magnesium, suclrtreated article being characterized by having a definite eutectic structure including such second minor constituent and by having an amount of such first minor constituent thus in solid solution in excess of that present in an untreated article of the same composition.

8. As an article of manufacture, a heat treated light metal alloy article, wherein magnesium is the predominant constituent, the alloy containing aluminum and copper as minor constituents, such treated article being characterized by having a definite eutectic structure including the copper constituent and by having an amount of aluminum in solid solution with the magnesium in excess of that present in an untreated article of the same composition.

9. As an, article'of manufacture, a heat treated light metal alloy article, wherein magnesium is the predominant constituent, the alloy containing as minor constituents a metal capable and another substantially incapable of forming a solid solution with the magnesium, such treated article being characterized by having a definite eutectic structure including such second minor constituent and byhaving an amount of such first minor constituent thus in solid solution in excess of that present in an untreated article of the same composition, such first minor constituent being substantially entirely thus in solution in such treated article.

10. As an article of manufacture, a heat treated light metal alloy article, wherein magnesium is the predominant constituent, the alloy containing aluminum and copper as minor constituents, such treated article being characterized by having a definite eutectic structure including the copper constituent and by having an amount ofaluminum in solid solution with the magnesium in excess of that present in an untreated article of the same composition, the aluminum being substantially entirely thus in solution in such treated article.

Signed by me this 8th day of January, 1927.

JOHN A. GANN. 

